Recently, since outstanding progress has been made in hot rolling techniques for manufacturing steel sheets and steel pipes, there has been a strong demand for increasing the properties of a hot rolling mill roll, in particular, for increasing wear resistance. In order to meet such a demand for increasing wear resistance, a high-performance roll (hereinafter, also called a high-speed steel roll), which has a surface layer having a chemical composition similar to that of high-speed tool steel and a microstructure including a hard carbide precipitated in order to significantly increase wear resistance, has been developed and put into practical use.
On the other hand, in the final finishing rolling stand for a steel sheet, cobble tends to occur on a rolled material and the surface skin pattern of rolls tends to be transferred to a steel sheet. Therefore, a nickel grain cast iron roll, which has high heat shock resistance as a result of containing graphite and has smooth surface skin, has been installed in order to perform hot rolling. However, in the case of a conventional nickel grain cast iron roll, there is a problem in that roll life is short due to poor wear resistance.
On the other hand, in the case of a high-speed steel roll having high wear resistance, there is a problem from the viewpoint of accident resistance, because, for example, a large heat shock crack occurs due to the occurrence of cobble or sticking. Therefore, in a stand in the latter stage of final finishing hot rolling of a steel sheet in which cobble tends to occur frequently, it is difficult to stably use a high-speed steel roll, and therefore a lot of nickel grain cast iron rolls have been used.
In order to solve such problems, for example, Patent Literature 1 proposes a hot rolling mill roll, in which wear resistance is increased by adding 1.0 to 5.0 wt. % of V to nickel grain cast iron. In addition, Patent Literature 2 proposes a hot rolling mill roll, in which wear resistance is increased by adding 2.0 to 8.0 wt. % of V to nickel grain cast iron and by forming, in terms of area ratio, 0.2% to 10% of MC type carbides in addition to 0.5% to 5% of graphite.
In addition, Patent Literature 3 proposes a surface layer material of a rolling mill roll, in which wear resistance and surface deterioration resistance are combined by decreasing the grain diameter of crystallized graphite in high-speed cast iron having a microstructure containing high-hardness compound carbides and graphite. Moreover, Patent Literature 4 proposes a wear-resistant hot rolling mill roll, in which occurrence of surface deterioration is suppressed by limiting the total content of Cr, V, and Nb to 3 wt. % or less and by forming, in terms of area ratio, 2% or more of crystallized graphite.
In addition, Patent Literature 5, Patent Literature 6, and Patent Literature 7 propose that wear resistance, sticking resistance, surface deterioration resistance, and heat shock resistance are increased by improving the form of MC type carbides through the addition of a combination of V and Nb and by controlling Cr content and effective C content. Moreover, Patent Literature 8 proposes the surface layer material of a hot rolling mill roll, in which heat shock resistance is notably improved by controlling a graphite structure in addition to the limitation on the chemical composition described above.